Restore link to list of packages in Base docs (#50353)

This link used to exist in the docs up to v0.2,
but it was removed in e91294f
as it was pointing to a doc page that was removed in
ef0c44d.

This change restores the link in the original place,
pointing to the up-to-date location.

doc/src/base/base.md

@@ -4,7 +4,8 @@
 
 Julia Base contains a range of functions and macros appropriate for performing
 scientific and numerical computing, but is also as broad as those of many general purpose programming
-languages.  Additional functionality is available from a growing collection of available packages.
+languages.  Additional functionality is available from a growing collection of
+[available packages](https://julialang.org/packages/).
 Functions are grouped by topic below.
 
 Some general notes: